1. Field of Invention
This invention relates to an under bump metallization structure of a semiconductor wafer. More particularly, the present invention is related to an under bump metallization structure of a wafer for enhancing the mechanical strength of the connection of the bonding pads to the wafer and the solder bumps.
2. Related Art
In this information explosion age, integrated circuit products are used almost everywhere in our daily life. As fabricating technique continue to improve, electronic products having powerful functions, personalized performance and a higher degree of complexity are produced. Nowadays, most electronic products are relatively light and have a compact body. Hence, in semiconductor production, various types of high-density semiconductor packages, for example ball grid array package (BGA), chip-scale package (CSP), multi-chips module package (MCM) and flip chip package (F/C), have been developed.
However, as mentioned above, flip chip is one of the most commonly used techniques for forming an integrated circuits package. Compared with a wire-bonding package or a tape automated bonding (TAB) package, a flip-chip package uses a shorter electrical path on average and has a better overall electrical performance. In said flip-chip package, the bonding pads on a chip and the contacts on a substrate are connected together through a plurality of bumps formed by the method of bumping process. It should be noted that there is further an under bump metallization structure disposed on the bonding pads of the chip to be regarded as a connection medium for connecting to the bumps and enhancing the mechanical strength of the connection of the chip to the substrate after said chip is attached to the substrate.
Referring to FIG. 1, it illustrates a partially cross-sectional view of a conventional semiconductor wafer 100. The semiconductor wafer 100 has a passivation layer 102 and a plurality of bonding pads 104 exposed out of the passivation 102. Besides, there is an under bump metallization structure 106, which is interposed between the bonding pads 104 and the solder bumps 108, regarded as a connection medium.
Referring to FIG. 1 again, as mentioned conventional under bump metallization layer 106 mainly comprises an adhesive layer 106a, a barrier layer 106b and a wetting layer 106c. The adhesive layer 106a is utilized to enhance the mechanical strength of the connection of the bonding pad 104 to the barrier layer 106b, wherein the material of the adhesive layer 106a is made of aluminum or titanium. The barrier layer 106b is utilized to avoid the diffusion of the underlying metal, wherein the material of the barrier layer 106b usually includes nickel-vanadium alloy, nickel-copper alloy and nickel. In addition, the wetting layer 106c, for example a copper layer, is utilized to enhance the wettability of the solder bump 108 with the under bump metallization structure 106. It should be noted that tin-lead alloy is usually taken as the material of the solder bump 108 due to its good mechanical strength of the connection of the chip to the substrate, when the chip included in the semiconductor wafer 100 is singulated into individual ones and then mounted on the substrate. However, lead is a hazard and poisoned material so lead-free alloy becomes the mainstream material of the solder bump 108.
Referring to FIG. 1 again, when the wetting layer 106c comprises copper or is only made of copper, tin provided in the solder bumps 108 is easily reacted with copper to form an inter-metallic compound at the duration of reflowing solder bumps 108. Namely, an inter-metallic compound, Cu6Sn5, is formed and interposed between the wetting layer 106c and the solder bumps 108. Besides, when the barrier layer 106b comprises nickel-vanadium alloy, nickel-copper alloy and nickel, tin provided in the solder bumps 108 further reacts with nickel provide in the barrier layer 106b to form another inert-metallic compound, i.e. Ni3Sn4, after the inter-metallic compound, Cu6Sn5, is formed. It should be noted that the inter-metallic compound, i.e. Ni3Sn4, formed between the under bump metallization structure 106 and the bumps 108 under the long-term reaction of tin and nickel has a plurality of discontinuous blocks, which make the solder bumps 108 peel off from the under bump metallization structure 106 more easily.
Therefore, providing another method for forming bumps to solve the mentioned-above disadvantages is the most important task in this invention.